Programmable digital filter implementation for loss-of-signal detection for serial communications applications

Abstract

A circuit generally comprising a first circuit and a second circuit. The first circuit may be configured to (i) detect a state of an input signal and (ii) present a plurality of intermediate signals each representative of the state of the input signal during a plurality of clock cycles. The second circuit may be configured to present a filtered signal in response to a selected number of the intermediate signals having a predetermined state.

1. An apparatus comprising:
 a sample circuit configured to (i) detect a state of an input signal and (ii) present a plurality of intermediate signals each representative of said state of said input signal during a plurality of clock cycles; and 
 a selection circuit configured to present a filtered signal in response to (i) a selected number of said intermediate signals having a lost state and (ii) a multi-bit selection signal representing a filtering value, wherein said filtered signal indicates said input signal has been lost when said selected number of lost states is greater than said filtering value. 
 
   
   
     2. The apparatus according to  claim 1 , wherein said sample circuit comprises:
 a detect circuit configured to (i) detect said state of said input signal and (ii) present a detected signal representing said state of said input signal; and 
 a plurality of shift registers configured to (i) sample said detected signal in each of said clock cycles and (ii) present said intermediate signals in parallel. 
 
   
   
     3. The apparatus according to  claim 2 , wherein said sample circuit further comprises a second shift register circuit configured to synchronize said detected signal to a clock signal defining said clock cycles. 
   
   
     4. The apparatus according to  claim 1 , wherein said selection circuit comprises:
 a plurality of logic gates each configured to present a second intermediate signal in response to at least two of said intermediate signals; and 
 a multiplexer configure to multiplex said second intermediate signals to present said filtered signal as determined by said multi-bit selection signal. 
 
   
   
     5. The apparatus according to  claim 4 , wherein each of said logic gates is configured to (i) receive one of said intermediate signals and one of said second intermediate signals and (ii) present another one of said second intermediate signals. 
   
   
     6. The apparatus according to  claim 1 , further comprising a second selection circuit configured to present a second filtered signal in response to a second selected number of said intermediate signals having a second predetermined state, said second selected number defined by a second multi-bit selection signal. 
   
   
     7. The apparatus according to  claim 6 , further comprising a status circuit configured to present a status signal indicating one of (i) a signal lost and (ii) a signal present to in response to both said filtered signal and said second filtered signal. 
   
   
     8. The apparatus according to  claim 7 , wherein said selected number and said second selected number are programmable. 
   
   
     9. The apparatus according to  claim 8 , wherein said selected number has a value different than said second selected number. 
   
   
     10. The apparatus according to  claim 9 , wherein said predetermined state comprises a loss-of-signal state and said second predetermined state comprises a signal present state. 
   
   
     11. A method of filtering an input signal, the method comprising the steps of:
 (A) detecting a state of said input signal; 
 (B) presenting a plurality of intermediate signals in parallel, each of said intermediate signals representing said state of said input signal during a plurality of clock cycles; and 
 (C) presenting a filtered signal in response to (i) a selected number of said intermediate signals having a lost state and (ii) a multi-bit selection signal representing a filtering value, wherein said filtered signal indicates said input signal has been lost when said selected number lost states is greater than said filtering value. 
 
   
   
     12. The method according to  claim 11 , further comprising the steps of:
 presenting a detected signal representative of said state of said input signal in response to detecting; and 
 sampling said detected signal in each of said clock cycles to present said intermediate signals. 
 
   
   
     13. The method according to  claim 12 , further comprising the step of synchronizing said detected signal to a clock signal defining said clock cycles in response to detecting. 
   
   
     14. The method according to  claim 11 , wherein step C comprises the sub-steps of:
 presenting a plurality of signals each in response to at least two of said intermediate signals; and 
 multiplexing said signals to present said filtered signal as determined by said multi-bit selection signal. 
 
   
   
     15. The method according to  claim 14 , wherein presenting said signals comprises the sub-step of performing a plurality of logical operations each receiving one of said intermediate signals and a one of said signals to present one of said signals. 
   
   
     16. The method according to  claim 11 , further comprising the step of presenting a second filtered signal in response to a second selected number of said intermediate signals having a second predetermined state, said second selected number defined by a second multi-bit selection signal. 
   
   
     17. The method according to  claim 16 , further comprising the step of presenting a status signal indicating one of (i) a signal lost and (ii) a signal present responsive to both said filtered signal and said second filtered signal. 
   
   
     18. The method according to  claim 17 , further comprising the step of programming said selected number and said second selected number. 
   
   
     19. The method according to  claim 18 , wherein said selected number has a value different than said second selected number. 
   
   
     20. A circuit comprising:
 means for detecting a state of an input signal, said state consisting one at a time of (i) a loss-of-signal state and (ii) a signal present state; 
 means for presenting a plurality of intermediate signals each representing said state of said input signal during a plurality of clock cycles; and 
 means for presenting a filtered signal in response to (i) a selected number of said intermediate signals having a lost state and (ii) a multi-bit selection signal representing a filtering value, wherein said filtered signal indicates said input signal has been lost when said selected number lost states is greater than said filtering value.